Conventionally, a distance measurement image sensor such as a CCD (Charge Coupled Device) image sensor and a CMOS (Complementary Metal Oxide Semiconductor) image sensor for using TOF (Time Of Flight) in which a distance can be measured by measuring flight time of light has been realized. Purposes of most of the conventional distance measurement image sensors are to measure the distance in a range from several meters to several to tens meters. A resolution of the measurement is from several millimeters to several centimeters.
Modulation methods of the distance measurement image sensors are generally classified into a pulse modulation system (refer to Non Patent Literature 1) and a sinusoidal wave modulation system (refer to Non Patent Literature 2). Both systems detect a signal, which depends on delay time of light, by performing lock-in detection in the sensor and calculate a distance based on the signals of a plurality of phases. In the pulse modulation system, a distance resolution is proportional to a pulse width of a light source and is inversely proportional to the square root of the number of stored electrons. In the sinusoidal wave modulation system, the distance resolution is inversely proportional to a frequency of sinusoidal wave modulation and the square root of the number of stored electrons. Accordingly, the resolution can be improved by shortening the pulse width of the light source and increasing the number of stored electrons or by making the modulation frequency high and increasing the number of stored electrons.